Hello DOBlake.
Thank you for your interesting post. I represent the Australian branch of the Buchanans, with my GGGrandfather James's sister Jane marrying Robert Sherrard. To be honest, I do not know whether you are related. On our side, as well as the side who immigrated to America, there is no David known as a name in the following generations. I cannot speak for anyone in Ireland.
Where I have researched since last posting information relates to trying to get back to Scotland. There is an interesting article written by a William Charles Reid in America. Look it up on Google. It is titled 'The Reids of Centreville, New Brunswick'. It discusses a marriage between an Elizabeth Buchanan to a Reid, and that the Buchanans were in Northern Ireland at around 1789. This fits, as my James Buchanan's father was born in Landaire (a corruption in spelling of Londonderry in about 1799-1801. A relative in America has his year of birth as 1799).
I believe, but have not confirmed, that this Reid family is related to our Buchanans. My James Buchanan took out land in Trewalla near Beaufort in 1865 next to a Robert Buchanan and a George Buchanan. Robert was the father of George. I am in contact with this Robert's GGranddaughter. This Robert came to Australia in 1853, at the same time my James arrived in Australia. This Robert and George immigrated from Dublin, Scotland, although it is not known whether they originated from Dublin. The connection is that this Buchanan family had a son whom they named Thomas Reid Buchanan. As far as I understand, this Robert Buchanan was related to my James Buchanan as a cousin. Certainly, my Buchanan relatives had contact with cousins in Scotland until the late 1950s/early 1960s. It is the Reid connection which links my Buchanans to Magheramore and Tamnaherin. I have also ascertained that my James Buchanan was born at Highmoor, Slaughtmanus. My Buchanans were definitely at Highmoor in 1825, and most likely there in 1820.
The problem with your David Joseph Buchanan in my branch of the family is the middle name Joseph. Due to the sentiments expressed in those days in my branch of the family, it is unlikely that there would be anyone in the family with the name Joseph.
I have isolated a relative in Magheramore and have his phone number. This is my next step. I tried to phone a last weekend, but I had the wrong area code. I am not familiar with Ireland. After that, I will be purchasing a month's worth of the Irish records and simply applying for any likely death certificate for a William Buchanan, husband of Mary Norris, who was the father of James, William, Elizabeth, Jane and the elusive Robert born c.1820. In the listings I have seen so far, there are about three William Buchanans who are potential candidates. The Buchanans live a long time.
I am interested to know which photos have similarities with your family of Buchanans. James and William Buchanan, the brothers who came to Australia, looked very different to each other. Some of William's children look a lot like his wife Matilda's family who came from Manchester.
I wish I could help you otherwise. If I find anything at all, even in the long term future, I will certainly be posting this information.